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Detection gross and Fine Leaks in Nuclear Fuel Elements I.I.Loktev, V.V.Rozhkov, A.B.Aleksandrov, I.G.Chapaev Novosibirsk Chemical Concentrates Plant (NZHK), Bogdan Khmelnitsky street 94, Novosibirsk, 630110, Russian Federation, E-mail: nccp@sol.ru Contact

Abstract.

Nuclear fuel tightness of light water reactor is a necessary requirement of their successful operation and safety. Therefore a series of strict non-destructive monitoring methods apply to constructional materials and joint welds of fuel elements on their production. Besides they use helium leak testing of all manufactured fuel rods. However this aspect of monitoring has the features, which need to be taken into account at its using.
The leak testing method has both low detection limit for fine leakage and upper limit for gross leakage. Each kind of leakage has its optimal testing conditions for detection. Sometimes preferably to carry out gross leaks testing by separate stage and may be done by another method, for example by manometric one.
Behavior of the leaky fuel rods of different types during manufacturing and operation is investigated completely. Therefore in some cases a fuel designers shell to carry out leak tests under elevated temperature. Hot leak detection has additional problems of suitability estimation of fuel elements. It is necessary to have temperature correlation for leakage flow rate.
The rigour requirements to fuel rod tightness and on the other hand the lack of a noticeable amount of rejected product provoke necessity of leak simulation and determination of their behavior under different conditions. There is special computer code developed by Moscow research institute VNIINM, which allow to do it in dependence of defect size. It is convenient to consider all questions of leaky fuel detection on base of conditional defect size which is equal to real one with same leak flow.
Some results on mentioned questions, obtained at Novosibirsk Chemical Concentrates Plant, which have allowed manufacturing of non-defective VVER fuel rods, are given in this report.

Role of a leak-checking time during defect detection.

They usually utilize helium as a trial gas, existing under cladding in finished fuel elements or passed though defect during special pressurization. When leaky products are in vacuum chamber they loose helium during preliminary pumping out and staying there before registration of leakage. Whether be detected the residuary gas in fuel elements depends on size of defect in the cladding.

It can be shown, that for any leaky product with interior volume V, helium pressure P₀, leakage flow Q_t will be defined for any time t from the equation:

(1)

In this expression the coefficient of exponential decay VP₀/Q₀ is a period of time which is necessary for gas flow out from fuel element with preliminary flow rate of Q₀. To estimate leak behavior in a time and choose the most convenient requirements of their detection, it is preferable to consider separately fine and gross leakage.

A fine leak has a small gas flow, which doesn't vary during some technological period of time T_tec, when fuel elements are in a manufacture stage. A gross leakage gives a large gas flow vanishing to zero during fuel production.
Next relations can describe these features:


a gross leaks		(2)
a fine leaks

Leaky element with VP₀/Q₀, approximately equal to T_tec, has leak with both kinds of mentioned features.
Expression (1) with preset t, for example t = T_tec, is a function of preliminary flow rate Q₀, having dependence on defect size, more exactly, on gas passing capacity. From this point of view we can introduce into practice the size of cylindrical defect with the same capacity for gas flow, which have real defect. It gives us possibility to use theoretical means linking gas flows, which we register, and cladding defect, which we try to detect. This way are used by number of researchers for analyze of leaky fuel element behavior /1-6/.
When we control tightness of fuel element with permissible leak flow q after T_tec from sealing, suitability for production can be determinate by the next expression:

(3)

A zones of defect detection

It is possible to show expression (3) graphically, not in scale, in figure 1 in axes "diameter of defect d" - "flow of leakage Q₀". The curve 1 shows magnitude of a leakage to the monitoring moment.
In conformity with expression (1) flow tends to zero in two cases, when initial flow of leakage Q₀ (as well as defect size) decreases to zero or increases to very large magnitude. It is shown in figure 1 that the method determines all defects in size range from d_min1 to d_max1.
The estimates define that these sizes range for VVER fuel rods is 0,1 - 10 microns. A darkened space under curve 1 (figure 1) is a working field for fine defect detection with initial helium in fuel elements as a trial gas.

The upper threshold of defect detection can vary from 1a to 1c if a period of time after sealing T_tec would change. It also can be higher after special pressing of the fuel elements by helium in chamber according to a course of dotted curves 2 in figure 1 up to d_max2 (approximately up to 100 microns). But, in case of only helium going in fuel rod during special pressing process the lower threshold of leaks detection can rise as well up to d_min2 (up to 2 microns). Hence, it is necessary to combine these both operations.

Defects with size more then d_max2 begin confidently be determined by other NDT methods: ultrasonic, eddy-current, X-ray. However, more reliable overlapping of detection fields in this region can be reached by another leak test method not having upper threshold of defect detection, - manometric method.
The figure 1 is divided on five zones concerning types of leak behavior and method of detection them.

Fig 1. The graph of flow and pressure change for leaky fuel rod depending on the defect size.

A combine leak-checking

In the figure 1 a curve 3 in axes "diameter of defect d" - "pressure P" displays not in scale a pressure change of capsulated helium under cladding of fuel rod depending on the defect size d during the technological time T_tec after sealing. NDT pressure detection method described for example in /7/, can define a pressure drop value after some T_tec, Sequence, it can find leaks in fuel element beginning from some Q_min, wich is estimated by expression:

(4)

where V and P_nom is volume and initial pressure of the fuel rod. Low leak detection threshold is defined by sensitivity of the device.
So, checking fuel element pressure is monitoring of important quality parameters of the fuel - value of pressure and presence of gross defects.
Obviously, Q_min>>q, and this manometric method of leak detection can not replace totally helium leak testing, but it reveals through defects from some d_man.(darkened space under curve). A manometric method has not upper threshold of defect detection. There is rather large region of overlapping with helium leak testing. As it shown on the figure a combined method of leak checking demands only pressure indicating, not measuring of it. This circumstance brings down requirements to sensitivity of a pressure-checking device.
Thus, during organization of quality production process on part of leak-checking as it is circumscribed above, necessary to have in mind:

• Part of fine leaks with defects smaller 0,1 micrometers will not be determined because of a small leakage flow, zone I on the figure 1, defects smaller 0,1 micrometers.
• Gross and fine leaks with defect size 0,1 - 10 micrometers will be determined confidently by helium leak-testing, zones II and III.
• Gross and fine leaks with defect size 2 - 10 will be determined by both methods (checking the initial helium and helium injected through defect by special pressing process, zones III and IV.
• Gross leaks with defect size 10 - 100 will be determined with special pressing process only.
• Part of gross leaks with defects bigger then 100 micrometers will be determined only by manometric method, zone V.

Comparison of equivalent and actual leak sizes

It is possible to estimate a consequences of replacement actual leak sizes by equivalent ones. A knowledge of defect size give us possibility to solve next tasks:

• definition of degree material untightness and opportunity of defect detection by any a means.
• estimation of a gas permeability,
• estimation of a water permeability,

The actual defect has a developed shape and can be considered rather slot then cylindrical channel. The degree of an aberration from the exact shape can be characterized by a ratio of the maximal and minimum sizes of a channel cross-section, or in another words, by ratio of a defect disclosing a to its width b. As a rule, a manufacturing defects have slot-type feature, for example crack in cladding, fig. 2, or welding with poor penetration, having the size relation close to 10. Depending on rate of splitting or kind of joining parts this relation can be close to 100.

Fig.2. Crack in a cladding.

Hence, it is possible to expect that, at least, one of the sizes of defect cross-section can be on the order or two more then equivalent diameter d.
Following to known expressions for gas flowing and fluids can be output relations between these parameters for:

fluid flowing and viscosity gas flowing :		(5)
molecular gas flowing :		(6)

For example, a crack on the figure 2 with disclosing 300 microns and width 3 mm has equivalent diameter 720 microns.
Magnitude of the disclosing defines an opportunity of defect detection by defectoscopy: X-ray, ultrasonic method, and can be appreciated too.

The performance of a gas permeability of a defect is given by its equivalent size by definition. Using gas lows it is possible to calculate an equivalent diameter of the channel in view of leak flow change in a time and parameters of Gas State. However it is necessary to take into account, that the change mode of gas flowing from viscosity to molecular is probable with diminution of a channel width.
The water permeability of the channel is connected with its capillary action, which inversely depends on smallest size of channel cross-section only .

So, the estimation allows consider that the performance of defects in product on base of equivalent diameter will be more conservative, concerning both material tight and leakage value estimation.

A role of a leak-checking temperature on defect detection

Behavior the leaky fuel rods of different types at manufacturing and operation are not clear and investigated completely. Therefore a fuel designers demand in some cases to carry out leak test under elevated temperature. Hot leak detection has additional problems of suitability estimation of fuel elements. It is necessary to have temperature correlation for flow rate of fine leakage.

As is shown by investigation /8/ a recalculations of molecular flow values, given, for example, at temperature T₀, must be made by formula (7) for any temperature T.

(7)

If tested workpieces are heated only in a defect zone, for example in a weld joint, the recalculations should be carried out by formula (8).

(8)

It is necessary to have in mind sharp magnification of desorption gases flow from metal items at the temperature higher then 150°C in vacuum.

A simulation of leakages

The rigour requirements to fuel rod tightness and lack of a noticeable amount of rejections provoke necessity of leak simulation and determination of their behaviour under different conditions. The Moscow All-Russia Research Institute of Inorganic Material (VNIINM) had developed mathematical code of prediction any scenario of behavior leaky items depending on an initial conditions and defect size /9/. Physically feasible simulate only gross leakage.

Expirience of fuel rod leak checking

All described above methods of leak checking are applied on Novosibirsk Chemical Concentrates Plant (NZHK):

• mass-spectrometric leak testing under elevated temperature with capsulation helium as a trace gas
• mass-spectrometric leak testing under room temperature with helium pressurized through defect as a trace gas,
• NDT of inside pressure.

Long-term manufacture and exploitation experience of the NZHK VVER fuel proves on the one hand non-defect production and, on the other hand, reliability of applied leak-testing techniques. It had been detected three kinds of through defects of fuel rods: cladding cracks, poor welding, porous end caps. A corresponding measures which was timely realized, helped to do non-defect production of nuclear fuel rods and exclude producing of leaking fuel even in single cases.

Conclusion.

Understanding of leaky fuel element behavior and particularities of leak-checking techniques, is important condition for reliability of production quality control.

Using of a helium leak-testing demands an estimation of leakage detection diapason. In case of multi-stage checking by different methods for exclusion of defect omission it is need overlapping or connection of detection diapason each methods at least. It can be made on base of defect size definition with using simulating code.

Reference.

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7. V.V.Pastoushin, A.Yu.Novikov, A.A.Ltmidov, "Thermophysical Automatic System for Non-Destructive Gas Pressure Inspection Inside Power Reactor Fuel Rods". 7th European Conference on Non-Destructive Testing, Copenhagen, 26-29 May 1998.
8. I.I.Loktev, S.A.Bezdenezhnih, Estimation of defect fuel rods detection under elevate temperature. Technical report NZHK 951-39-85, 1985.
9. E.V.Dmitrievskaya, S.I.Sorokin, Calculated date for helium leak testing. Rotaprint VNIINM. 1984.

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